The cooling systems of liquid-cooled combustion engines in vehicles are usually closed positive-pressure systems, comprising an expansion tank. This expansion tank has to fulfil several important tasks: it must compensate for the volume changes resulting from the temperature changes of the cooling liquid, it must collect the air present in the liquid system, and it must prevent excessive depression or cavitation in front of the pump. These expansion tanks are installed in parallel with the main liquid circuit. The expansion tank is so connected that the vent pipe, starting from the highest geodetic point of the cooling system, discharges into the expansion tank and the fall pipe from the expansion tank is connected immediately ahead of the pump intake of the liquid pump. This arrangement is known primarily with utility vehicles but is not completely satisfactory. The compensation of volume changes does not create any difficulties; it is sufficient to maintain a certain container volume and a certain liquid level. In the simplest automatic venting system a permanent connection is provided between the main liquid circuit and the expansion tank via the vent tubes. The air segregated at the geodetic high points of the liquid circuit can move uninhibited into the expansion tank. In cooling circuits with a single geodetic high point the venting process can be accellerated by a centrifugal separator described as in the British Pat. No. 1,497,988, which is advantageously placed at the geodetic high point of the main liquid circuit. Since in most cases it is required to vent at several geodetic high points, the applicability of such centrifugal separators is limited because the installation of several of these units creates high flow resistance with the relatively large amounts of liquid flowing through the ventilation pipes. Therefore, in the case of several geodetic high points a vent pipe runs from each high point to the expansion tank. Advantageously, the inner diameter of the ventilation pipe is made as large as possible, first because of the danger of clogging by impurities and scale deposits, and second to provide sufficiently rapid ventilation. Since during the functioning of the engine various different pressures will prevail at the several geodetic high points the vent pipes practically have to be individually connected to the expansion tank. Thus the amount of liquid flowing into the expansion tank can rise to such an extent that an unduly high liquid velocity in the fall pipe is necessary. In order to maintain this rapid liquid flow in the fall pipe, a large static pressure differential is required, such that it could be larger than the static pressure difference resulting from the level distance between the level of the liquid in the expansion tank and at the connection point of the fall pipe in front of the liquid pump. In this case the expansion tank becomes ineffective as a means for maintaining the static overpressure in front of the impeller of the liquid pump, thereby resulting in vapor formation and cavitation.
Technical solutions are known for overcoming the contradictory requirements of venting and of maintaining a desired static pressure at the pump.
The simplest solution is the installation of a closure construction (valve, cock etc.) in the vent pipe. With this closure construction the flow of liquid in the pipes or the expansion tank, respectively, can be interrupted after the ventilation, and in the immediate vicinity of the intake pipe of the liquid pump a static pressure can be achieved, which is larger than the pressure prevailing in the vapor space of the expansion tank and larger than the pressure resulting from the liquid level difference already mentioned. The solution has the disadvantage of requiring action by the human being and improper handling results in the possibility that air remains in the liquid system, which with nonstationary operation can cause cavitation, and reduce on the water side the circulated amount of liquid and the heat transfer. When the closures remain open the first-mentioned problem remains unchanged.
These disadvantages can be avoided in accordance with the technique described in German Patent DE-PS No. 1,931,918 by providing only a portion of the venting lines with manual closure constructions, whereas in the other portion the continuous liquid flow parallel to the main circuit through the expansion tank is assured. This apparatus is not automatic; it requires intervention by man.
It is furthermore disadvantageous that even after closure of the valve a considerable amount of liquid circulates and furthermore that a part of the air, which accumulates after the filling of the system with liquid during the engine operation, remains in the system because of the closed valve. In order to clear this, the motor has to be stopped, the valve has to be opened again, and the missing liquid has to be added. Only after these procedures have been repeated several times is the desired filling state achieved, and this depends to a large extent on the skill of the person performing the venting.